In computer tomography and NMR imaging, first data for image formation on a detected body is acquired by scan using X-ray or nuclear magnetic resonance, and subsequently an image is reconstructed by a designated algorithm using these data. In case of continuously scanning the detected body plural times to acquire a plurality of scan data and reconstructing an image one by one according to the respective scan data, data acquisition and image reconstruction are conducted in parallel by a pipe-line system to achieve speeding up of the operation. At that time, as shown in FIG. 4, a large-capacity buffer memory 2 (intermediate buffer) comprising a magnetic disc device or the like is interposed between a data acquisition device 1 and an image reconstruction device 3 to absorb a difference in speed between the data acquisition operation and the image reconstruction operation. The reconstructed image is displayed on a display device 4. In the thus constructed apparatus, reconstruction of an image can not be started by the image reconstruction device until data for one scan at least is collected in the intermediate buffer, so that the time spent until the first reconstructed image is made longer for that. On the contrary, according to Scan & Recon system, image reconstruction is started simultaneously with the start of scan, so that the time spent until the first reconstructed image is obtained is made shorter. In this system, however, the next scan can not be started before image reconstruction is completed. Accordingly, this system is not suitable for use in continuously scanning plural times at high speed.